TORONTO (Reuters) - Twice as much autumn rain and early
winter ice helped Lake Superior, the biggest of North America's
Great Lakes, bounce back from record low water levels reached
last year.

The deep, cold lake on the Canada-U.S. border -- the
largest freshwater body of water in the world by surface area
-- rose about 31 cm (1 foot) in seven months, with half of that
in April alone as the spring thaw melted heavy winter snowfall
that arrived late in the season.

The turnaround in the uppermost of the Great Lakes could
literally trickle down to its four lower cousins, spelling
relief for shippers who use the major waterway and residents
concerned over shallow channels and receding shorelines.

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"The spring runoff was much anticipated, and conditions
have appeared to return to normal," said Melissa Kropfreiter, a
hydraulic engineer with the U.S. Army Corps of Engineers, which
studies the water levels.

In the last 30 years, precipitation has decreased while
evaporation has increased. That led to higher water
temperatures and, in recent years, lower water levels in the
three upper Great Lakes -- Superior, Huron and Michigan.

With the inland waterway a key route for shipping bulk
commodities like grain, steel or coal, the low water forced
ships to lighten their loads. Last summer, some of the shallows
and riverbeds used by fish for spawning dried up.

But that pattern, seen by many as a mark of global climate
change, appears to have reversed at least over the last half
year.

HURON, MICHIGAN ALSO RISING

Lake Huron and Lake Michigan, which share the same water
level, also rose through the winter and spring as 50 percent
more snow fell in a region that includes the U.S. states of
Wisconsin and Michigan, and parts of the Canadian province of
Ontario.

Ice on the water curbs winter evaporation, helping to
maintain levels. But much of the snowpack evaporates before the
spring thaw and never reaches the lakes, which may explain why
Huron and Michigan are still slightly lower than last year,
experts say.

"If there are a lot of cool, sunny days, the snow goes
straight into water vapor," said Ralph Moulton, a senior
engineer at Environment Canada who studies Canada-U.S. boundary
waters.

The new data on water levels comes as the International
Joint Commission, an independent body formed by the U.S. and
Canadian governments, studies whether dredging on the St. Clair
River has contributed to declining levels on the upper lakes.

The St. Clair River is a major shipping route near Detroit
that connects the lower lakes of Lake Erie and Lake Ontario
with the upper trio.

While the upper lakes remain below their long-term
averages, water levels in the smaller, lower lakes are above
average.

The massive waterway connects to the Atlantic Ocean through
the St. Lawrence Seaway, a system of locks and canals that
opened in 1959, allowing ocean-going vessels into the
industrial heartland of North America.

The largest ships on the Great Lakes forfeit some 267
tonnes of cargo for every inch of water level lost, said Stuart
Theis, executive director of the United States Great Lakes
Shipping Association.

"Every inch lost is revenue the shipping companies can't
get out of a trip, and can't get back."